Contact screen and printing member produced therefrom



May 3 1966 F. J. EEKHOUT 3,249,437

CONTACT SCREEN AND PRINTING MEMBER PRODUCED THEREFROM Filed Nov. 25,1964 5 Sheets-Sheet 1 lllllllllllllllllllllmml INVENTOR. FREDERIK d.EE/(hOl/T F. J. EEKHOUT May 3 1966 CONTACT SCREEN AND PRINTING MEMBERPRODUCED THEREFROM Filed Nov. 23, 1964 3 Sheets-Sheet 2 FkEDEEIK J EEKHOUT INVENTOR. Mam

ATTOR/IEKS BY ,0 .a/

F. J. EEKHOUT May 3 1966 CONTACT SCREEN AND PRINTING MEMBER PRODUCEDTHEREFROM Filed NOV. 25, 1964 3 Sheets-Sheet 3 "ll-HUME @557 UnitedStates Patent 3,249,437 CONTACT SCREEN AND PRINTING MEMBER PRODUCEDTHEREFROM Frederik Johannes Eekhout, Segbroeklaan 526, The Hague,Netherlands Filed Nov. 23, 1964, Ser. No. 414,514 Claims priority,application Netherlands, Mar. 5, 1959, 236,806 5 Claims. (CI. 96-45)This application is a continuation-in-part of Application Serial No.12,541 filed March 3, 1960, now abandoned.

The present invention relates to a contact screen for use in making aprinting block from a picture having areas of optical density, and to aprinting member, such as a printing block, produced by using the contactscreen.

The contact screen of the present invention is of the type of contactscreen having groups of small, light-transmitting areas, each groupbeing constituted by areas of diiferent optical densities and ofcongruent shape, and each group having light-transmitting areas of alldensities. The light-transmitting areas adjoin one another and adjoiningareas of adjacent groups, and each group has sub-groups the areas ofwhich have an identical optical density. The densities of saidsub-groups are different from each other.

This type of contact screen is described in French Patent 336,056, andcan be employed for the reproduction of a picture of an object to 'bedepicted in the form of one or more series of dots, all dots of oneseries having the same color and the same degree of transmission orreflection, the total area of the dots of a series in a certain portionof the picture being a function of the relative brightness of thecorresponding portion of the object. The printed dots of a series allhave the same size, the number of dots in a given portion of the picturebeing such that this portion of the picture has the desired overalldegree of reflection or transmission.

In the performance of such a method by means of this known contactscreen, a number of difiiculties are encountered.

Since this known contact screen is manufactured by superposing bands ofmaterial with certain optical densities in various patterns, the opticaldensities of the lighttransmitting areas, as far as their mutualdifferences are concerned, are determined by the selection of theoptical densities of the single bands used. A second limitation of thisprior art contact screen is that the number of density levels which canbe present in the screen is determined by the number, the mutualdifference in shape and optical densities, and the possibilitiesregarding the geometrical arrangement, of the bands. The number oflevels is also limited because the geometrical configuration cannot beextended at will. If, for example, more than three bands are used, agreater number of areas of higher densities is indeed obtained, but onthe other hand more and more areas of lower densities are lost. A fourthrestriction of the prior screen is in the number of areas of the sameoptical density within one group. In the prior art screen, this numberis determined by the number, the optical value, and the. location ofthe. bands. A fifth limitation of the prior screen is in theinterrelation of the location of the areas of different opticaldensities. As a matter of fact, this is entirely determined by thearrangement of the bands. The result of these five limitations on thefreedom of using the prior contact screen is that the distribution ofthe optical densities in the screen plate cannot be adapted to thedemands to be made in connection with a true representation of thedifferences in brightness of the object to be depicted. Hence it is, inparticular, no-t pos- 3,249,437 Patented May 3, 15366 "ice sible toobtain an adaptation to the characteristics of the photographic andprinting means employed. A further consequence of the relatively limitedarrangement of the areas of different densities is that there is anincreased chance of the printed dots in the printed picture adjoiningeach other.

It is an object of the present invention to provide a contact screenwhich does not have the limitations of the prior art screen.

It is an object of the present invention to provide a contact screen inwhich the densities of the steps, the number of steps, the number ofareas per density value, and the relative positions of the areas may beselected so that any desired adaptation of the negative to be reproducedcan be obtained.

To this end, the contact screen according to the invention ischaracterized in that number of sub-groups and their densities and thenumber of areas in each sub-group can be selected without anylimitations imposed by the fact that the screen is a configuration ofintersecting bands.

An additional degree of freedom can be obtained, according to theinvention, by varying the relative positions of the areas in the variousgroups. This enables a further adaptation to the requirements of aspecific reproduction to be depicted.

Since the optical densities can be freely selected as far as their valueis concerned, it is of importance, and it is possible according to thepresent invention, so to select the densities of the various areas suchthat the said densities constitute an arithmetical series, the logarithmof the relative number of areas of a given density being at least asubstantially linear function of the said density.

It is of particular advantage for the light transmitting areas to havethe shape of a regular hexagon.

The invention also relates to a printing block made by means of acontact screen as described herein.

The invention will be illustrated hereinafter with reference to theaccompanying drawings, in which:

FIG. 1 is a section, on enlarged scale, through a negative, aphotographic positive material and a screen according to the invention;

FIG. 2 is a graph showing the relationship between the magnitudes ofvalues which play a part in the method and in the screen according tothe invention;

FIG. 3 is a greatly enlarged plan view of a preferred shape of the smalllight transmitting areas of the screen according to the invention;

FIG. 4 is an illustration of one way in which the contact screen can bebuilt up; and

FIG. 4A is an enlarged view of part GI of FIG. 4 showing the screen withthe areas of different densities represented therein.

In the method of reproducing a negative using a screen according to theinvention, the same steps can be used as those for conventionalscreening methods, the contact screen according to the invention beingused in the same step as the well-known screens formed by engravedoptical glass. However, the particular adjustments and auxiliarytreatments, which are necessary when using the wellknown screens, arerendered superfluous by using the screen of the present invention. Theinvention, however, can be best understood by means of a specificexample, the example here being the case where a transparent black andwhite negative picture is translated into dots (a screened picture) on aphoto-sensitive material. Hereinafter the transparent black and whitenegative will be called the negative for short and the picture thereofmade on the photo-sensitive material will be called the positive.

In FIG. 1 the numeral 1 represents the negative and 2 the positive, bothin section. The numeral 3 designates a contact screen according to theinvention, the construction and the operation of which Will be explainedhereinafter. As appears from this figure, the negative comprises anumber of areas of different transmissivity. The transmissivity can beexpressed as 1-=I/I (also called transmission factor or in the case ofreflection the reflection factor), in which 1,, is the amount ofincident light and I the amount of transmitted light. Inasmuch as thehuman eye sees as differences in luminous intensity what actually areratios of luminous intensity, it is in many cases advisable to expressthe blackening of the photographic material by the density D=log 1/1=log1'; D= therefore corresponds with T=1-0 or 100%, D=1 corresponds withr=0.l or D=2 corresponds with 'r=0.0l or 1% etc. The transmissivityratios are converted into differences in density by the use oflogarithms, which is in accordance with the impressions of the humaneye.

In order to obtain a true reproduction of the negative 1 on the positive2 in the case under consideration, the distribution of densities in thenegative must be represented in the positive in a reverse sense, andaccording to the invention this must be effected by means of a patternof dots, which consists of dots of equal size, the areas havingdifferences in photographic density being reproduced in the'positive byareas having differences in the relative number of dots.

The positive material 2 has a very high gamma-value, which means thatwhen a certain exposure dose is reached and then exceeded, a heavyblackening is obtained sub stantially immediately, whereas below suchthreshold dose no blackening takes place. Furthermore it is assumed thatthe density range i.e. the differences between the greatest and theleast density in the negative amounts to 2.00 and that during theprinting the exposure dose will be such that for a negative densityD=2.00 the threshold dose at which blackening of the positive materialwill take place is just not reached.

Seeing that the half-tone positive is afterwards converted into aprinting means (e.g. a half-tone block or an offset plate) Which is usedfor printing the ultimate picture, the positive 2 may be regarded hereas if it were replaced by the printed image.

The object of the method of reproducing the negative is to have thepositive and thus the printing means formed therefrom divided into dotsin, such a manner that the number of dots per unit of surface areadetermines the density of the relevant portion of the image. To achievethis end the relationship between the number of dots and the imagedensity resulting therefrom is first established. Let us suppose that ablack printing ink is used which has a density D=1.50, so a reflectionor transmission factor 'r =0.032, while the paper has a reflectionfactor r =1.00 (D=0.00). If the relative area covered by printed dots,i.e. the number of printed dots actually in the area in question dividedby the number of dots that could possibly be printed in'the area, isequal to n, the remaining white area having no dots thereon has a sizeof (ln). The total reflection factor then is and in general The numberof dots for obtaining a certain value of 1- is hence proportional to 1T,and is even substantially equal thereto.

According to the present invention, the contact screen 3 which is usedcomprises a large number of small areas 4 of equal size and of differenttransmissivity, which areas are arranged in mutually identical groups,each containing sub-groups of areas having the same densities, and thediflferent sub-groups in a group of areas having different density suchthat each group has areas of all transmissivities that are employed. Ina preferred embodiment according to the invention, the densities of thesubgroups of areas within a group form an arithmetical series; in thesubjoined example it is assumed that the difference in density betweenthe successive densities of the subgroups amounts to 0.20, the extremevalues being 0.00 and 2.00.

The negative as. shown in FIG. 1 is likewise divided into zones betweenwhich there is a difference in density of 0.20, the lowest and highestdensities being 0.00 and 2.00 respectively. It has already been statedthat in th example under consideration the exposure dose given to thenegative is such that the light transmitted by the zones of the negativewhere D=2.00 is not quite sufiicient to effect blackening of thepositive, so that even in combination with the fully transparent areasof the screen where D=O.00 and the light transmitted by the negativethrough the zones where D=2.00 is insufficient to produce a black spotin the positive. The zones of the negative where D=l.80 will transmitsuflicient light to cause blackening of the positive behindthe screenareas where D=0.00, but not sufficient to cause a blackening behind theareas of the screen where D=O.20 orgreater. Accordingly, in the zones ofthe positive which correspond with the zonesof the negative whereD=l.80, a number of black dots will form which number is equal to thenumber of screen areas where D=0.00; the picture in these zones musthave a density D=0.20 to have a difference in density from the precedingzones in which no dots are printed at all which is the same as thedifference in-density between the correspond-ing portions of thenegative. The zones of the negative where D=l.60 will transmitsuflicient light to produce a blackening behind the screen areas whereD=0.20 as well as a blackening of the positive behind the screen areaswhere D=0.00. Accordingly, in the corresponding zones of the printedimage, there is present a number of black dots that is equal to the sumof the number-of screen areas where D=0.00 and D=0.20. Any subsequentzone of the negative of lower density will form a new group of dots inthe positive, which dots contribute towards the blackening as isintended.

It is still necessary todetermine how many screen areas of a certaintransmissivity are required to eifect the desired blackening in thescreened positive. It follows from the foregoing that a zone of thepositive having a certain blackening contains a number of dots which isequal to the number of dots in positive portions that are less blackplus a number of additional dots, which latter number equals the numberof areas of the screen of the greatest density that is still operativein this zone of the positive. The number of additional dots necessary toproduce each successive density level can be determined, therefore, asthe difference between the total number of dots of said level and thenumber of dots which produce the preceding level.v The relationshipbetween the reflection factor of a portion of a printed image and therelative number of dots required therefore has already been establishedhereinbefore [Formula 1)]. Said number of dots, naturally, is equal tothe number of black dots in the positive which was used formanufacturing the printing means. However,.there is a difference betweenthe extent of blackening of the positive and that of the printed image;for the printing ink used has, as was assumed, a density of 1.50, sothat the maximum density. and hence the degree of blackening achievablelikewise amounts to 1.50. Because in the printed image the same numberof density levels must be present as in the negative, the difference indensity between said levels in the printing means amounts to 0.15instead of 0.20.

The subjacent table is a tabulation of the calculation outlined above;in this table Dimax is the density of the screen areas that are justoperative and n, the additional number of said areas necessary for thatdensity as compared to the area of the next preceding density level.

Due: Dos vrint or-int D Linux. 7

2. 0. U0 0. 0O 1. 000 0. 000 1. 8O 0. 20 0. 15 0. 708 O. 302 O. 00 0.302 1. 60 0. 40 0. 30 0. 501 O. 516 U. 20 U. 214 1. 40 O. 60 0. 45 0.355 U. 666 0. 40 0. 150 1. 2O 0. 80 0. 6O 0. 251 O. 774 0. 6D 0. 108 1.(l0 1. 00 U. 75 0. 178 O. 849 0. 80 O. 075 0. 80 1. 20 O. 90 0. 126 0.903 1. 00 O. 054 0. (i0 1. 40 1. U 0. 089 0. 941 1. 2O 0. 038 0. 40 1.60 1. 2O 0. 063 U. 968 1. 40 0. 027 0. 20 1. 80 1. 35 0. 045 0. 987 1.(i0 0. 019 D. 00 2. 00 1. 50 O. 032 1. 000 1. 80 0. 013

FIG. 2 is a graph in which the uppermost curve, the dots-dash curve,shows the relationship between the relative number n of printed dots andthe density provided thereby. The dashed-line curve shows therelationship between the relative number of n, of additional dots andthe printed density, and the full-line curve shows the relationshipbetween it, and the density of a given set of screen areas. Thelast-named curves turn out to be straight lines, the inclination of thelatter being equal to -0.74; so that the following equation holds good:

or in general For a different number of subdivisions of the density ofthe screen areas or another density of the printing ink, thecoeflicients a and b can be calculated in a corresponding manner.

. It has been indicated by means of arrows in the graph that thecalculated values of n, are equal to the corresponding densities of theprint and the screen. The associated densities dilfer, it is true, butthey can be simply deduced from the calculating method followed.

If instead of a negative-positive method a reversal process is used, aslightly different line of reasoning must be followed. In reversalprocesses a negative image is formed first which by chemical treatmentand by re-exposure is reversed into a complementary positive image. Thatis to say, if said provisional negative image was screened, the positiveimage would have a number of screen do-ts complementary to the number ofdots in the negative. Consequently, the contact screen used with theprovisional negative is to be adapted to produce these complementarynumbers of dots.

As regards the shape of the screen areas, it should be noted that it ispossible to use all geometrical shapes which can be arranged inadjoining relationship without any interspaces being left, examples ofsuch shapes being triangles, squares and hexagons. The latter shape isto be preferred, because owing to the alternating arrangement of thefaces, the chance that lines will be seen in the screen pattern issmaller than if squares are used, while moreover the hexagon mostclosely approaches a circle. Such a pattern is shown in FIG. 3, thefaces having not yet been provided with the required grayings. Theformation of groups may be effected in different manners. A hexagonalcircumference of the groups is most favorable owing to the symmetry andthe optimum filling of a surface having a minimum area.

Though generally speaking the areas adjoin each other, it may sometimesbe desirable to provide a division between the faces. This may beeffected by means of an opaque edge between the faces.

The size of the screen areas depends on the demands which are made onthe reproduction and the lower limit of said size is determined by thesurface properties of the paper to be printed. It is possible to achievea diameter of 0.05 mm. and even less.

The calculation described above holds good for those cases where thedifferences in density remain unchanged during the steps of thetreatment prior to the screening operation and where it is not necessaryto change the number of dots in certain or in all of the density zones.It will be clear that such changes can be accommodated in the givencalculating scheme without difiicutly, and that therefore the contactscreen can easily be adapted to special treatments and materials. It isalso clear that what holds good for a black and white reproductionadmits of being simply transferred to multi-color printing methods.

FIG. 4 is will be seen that there are two vertical columns of figures,the left column containing 7 figures and the right column 6. Furthermorethere is a subdivision into horizontal rows which from the top downwardsare designated by the letters AG inclusive. Consequently, the figure inthe bottom right hand corner will be designated by GII etc.

FIG. 4 is a greatly enlarged portion of a contact screen. Adjacentportions of the contact screen will generally be built up in the samemanner, i.e. according to the same pattern, but this is in principle notnecessary. All areas of the sub-group of dark areas contained in thegroup of areas in FIG. AI, thirteen in this example, have the samephotographic density. The same applies to the darkened areas forming thesub-groups of areas shown within a group of areas the same size as thegroup of areas in FIG. AI in the right column, i.e. sub-group B2(containing twelve areas) to sub-group G2, inclusive. Whereas thephotographic density of the areas forming a sub-group is the same forall areas, of the sub-group, the photographic density of the overallgroup of areas increases step-wise from AI to G2. In each of the FIGS.AI, B2 to G2, the areas of the sub-group are uniformly distributed overthe space occupied by the overall group of areas. This distribution issuch, however, that as each of the various sub-groups is superposed uponthe preceding sub-groups, the darkened and undarkened areas form aregular, and preferably homogenous, pattern. Thus, for example, group AIhas a regular pattern as does group B2. When these two groups superposedone upon the other, they form the FIG. B1, which again has a regular andhomogenous pattern, i.e. a pattern which has no predominant orientation.If the regular pattern C2 is superposed on the pattern of FIG. B1, theregular and homogenous pattern Cl is formed. It should be noted that ifthe blackened areas are going to adjoin each other, which in the exampleshown occurs for the first time in FIG. E1, the white areas becomeseparated, again in a regular and homogenous pattern.

It should be borne in mind that the embodiment shown in this figure ismerely an example. As shown, FIG. AI has thirteen areas forming thesub-group. It would also be possible, however, to begin with a sub-groupof five areas, namely, one in each corner and one in the center. It willbe evident that this would also necessitate a change in the pattern ofthe succeeding sub-groups having different densities.

One of the advantages of the method and the preferred embodiment of thecontact screen according to the invention is that in the case ofmulti-color printing it is not necessary to change the screeningdirection, for the chance of a visible moir-effect is much less thanwith the conventional screen pattern, the pattern according to thepresent invention containing many more dots.

It is also an advantage that the contact screen can be adapted in asimple manner to the materials used or to produce particular effects.

In the above example the positive will acquire the same photographicdensity as the negative; however it Will be clear that the distributionof dots may be so effected that the photographic density of the screenedelement, i.e. the positive, is greater or less than that of e thestarting material, i.e. the negative.

Although in the foregoing the basic idea of the invention, namely theuse of screen dots of an equal area but varying in number, has beendescribed with reference to a photographic method, said basic idea isexplicitly not limited to photographic methods. The invention can alsobe applied to electronic devices which employ light spot scanning andwhich directly provide the screened printing means. Hitherto saiddevices have always used a variable dot size, but they can be adapted tooperate with a variable number of dots.

The contact screens according to the invention may be made of anysuitable transparent material e.g. glass, cellulose tri-acetate, and thelike. As a rule it is preferred to use a contact screen the flexibilityof which is opposite to that of the photographic printing material, i.e.in the case of photographic plates, a film screen, and in the case offilms, a plate screen, so as to ensure a good contact and therefore anabsence of interference figures. Because of the fact that in the graphicart vacuum cassettes are conventionally used, this is not an objection.

Finally the contact screens according to the invention can be made in :arelatively simple manner by first drawing or printing a master screen ona greatly enlarged scale and blackening the areas into which said screenis to be subdivided according to the required pattern. Subsequent- 1ysaid screen is photographically reduced. The degree of blackening of theareas of the master screen is to be adapted to the density curve of thephotographic material which, if said curve is accurately known and ifthe reflection and transmission factors of the blackening agents used inthe master screen have been accurately measured, does not give rise tospecial difliculties. If the color-sensitiveness of the photographicmaterial used is accurately known, it is also possible to provide theareas of the master screen with such different colors that the desiredblackening of the photographic material is obtained,

1 claim:

1. A contact screen for forming a printing means for printing a picture,said screen having identical groups of small light transmitting areas,each group being made up of congruent areas, said light transmittingareas adjoining one another and the areas on the edges of the groups inparts of the screen other than the edges thereof adjoining areas ofadjacent groups, each group of areas having subgroups of areas with thephotographic density of each area in a subgroup being identical to thephotographic density of the other areas in the subgroup, the densitiesof the areas in the respective subgroups being different from thedensity of the areas in each of the subgroups within said group of areasand there being a subgroup corresponding to each photographic density ina series of photographic densities, the areas of each subgroup beingdistributed over the total area ofthe corresponding group in a regularhomogeneous optical pattern and the areas of each subgroup within agroup together with the areas of the other subgroup-s within the grouphaving a lower photographic density forming a regularhomogene-ousoptical pattern.

2. A'contact screen as claimed in claim 1 in which the arrangement ofthe areas in the groups is mutually different.

3. A contact screen as claimed in claim 1 in which the densities of thedifferent subgroups of areas Within a group are in an arithmeticalseries, and in which the logarithm of the relative number of areas in asubgroup is at least a substantially linear function of said density.

4. A contact screen as claimed in claim 1 in which said lighttransmitting areas have the shape of a regular hexagon.

5. A printing member for making a picture of an object to be reproducedin the form of at least one series of dots, all dots of a series havingthe same color and the same degree of transmission or reflection and thetotal area of the dots of the series in the respective portions of theimage of the object being functions of the relative brightness of thecorresponding respective portions of the object, the said printingmember being made by exposing a base having a photosensitivelayerthereon to the picture to be reproduced through a contact screen,said screen having identical groups of small light transmitting areas,each group being made up of congruent areas, said light transmittingareas adjoining one another and the areas on theedges of the groups inparts of the screen other than the edges thereof adjoining areasadjacent groups, each group of areas having sub-groups of areas with thephotographic density of each area in a sub-group being identical to thephotographic density of the other areas in the subgroup, the densitiesof the areas in therespective subgroups being different from the densityof the areas in each of the other sub-groups within said group of areas,and there being a sub-group corresponding to each photographic densityin a series of photographic densities, the areas of each sub-group beingdistributed over the total area of the corresponding groups in a regularhomogeneous optical pattern and the areas of each sub-group within agroup together with the areas of the other sub-groups within the grouphaving a lower photographic density forming a regular homogeneousoptical pattern, and chemically treating the base to make a printingmember thereof.

References Cited by the Examiner UNITED STATES PATENTS 805,244 11/ 1905Szczepanik 9645 NORMAN G. TORCHIN, Primary Examiner,

A. D. RICCI, Assistant Examiner.

1. A CONTACT SCREEN FOR FORMING A PRINTING MEANS FOR PRINTING A PICTURE,SAID SCREEN HAVING IDENTICAL GROUPS OF SMALL LIGHT TRANSMITTING AREAS,EACH GROUP BEING MADE UP OF CONGRUENT AREAS, SAID LIGHT TRANSMITTINGAREAS ADJOINING ONE ANOTHER AND THE AREAS ON THE EDGES OF THE GROUPS INPARTS OF THE SCREEN OTHER THAN THE EDGES THEREOF ADJOINING AREAS OFADJACENT GROUPS, EACH GROUP O AREAS HAVING SUBGROUPS OF AREAS WITH THEPHOTOGRAPHIC DENSITY OF EACH AREA IN A SUBGROUP BEING IDENTICAL TO THEPHOTOGRAPHIC DENSITY OF THE OTHER AREAS IN THE SUBGROUP, THE DENSITIESOF THE AREAS IN THE RESPECTIVE SUBGROUPS BEING DIFFERENT FROM THEDENSITY OF THE AREAS IN EACH OF THE SUBGROUPS WITHIN SAID GROUP OF AREASAND THERE BEING A SUBGROUP CORRESPONDING TO EACH PHOTOGRAPHIC DENSITY INA SERIES OF PHOTOGRAPHIC DENSITIES, THE AREAS OF EACH SUBGROUP BEINGDISTRIBUTED OVER THE TOTAL AREA OF THE CORRESPONDING GROUP IN A REGULARHOMOGENEOUS OPTICAL PATTERN AND THE AREAS OF EACH SUBGROUP WITHIN AGROUP TOGETHER WITH THE AREAS OF THE OTHER SUBGROUPS WITHIN THE GROUPHAVING A LOWER PHOTOGRAPHIC DENSITY FORMING A REGULAR HOMOGENEOUSOPTICAL PATTERN.
 5. A PRINTING MEMBER FOR MAKING A PICTURE OF AN OBJECTTO BE REPRODUCED IN THE FORM OF AT LEAST ONE SERIES OF DOTS, ALL DOTS OFA SERIES HAVING THE SAME COLOR AND THE SAME DEGREE OF TRANSMISSION ORREFLECTION AND THE TOTAL AREA OF THE DOTS OF THE SERIES IN THERESPECTIVE PORTIONS OF THE IMAGE OF THE OBJECT BEING FUNCTIONS OF THERELATIVE BRIGHTNESS OF THE CORRESPONDING RESPECTIVE PORTIONS OF THEOBJECT, THE SAID PRINTING MEMBER BEING MADE BY EXPOSING A BASE HAVING APHOTOSENSITIVE LAYER THEREON TO THE PICTURE TO BE REPRODUCED THROUGH ACONTACT SCREEN, SAID SCREEN HAVING IDENTICAL GROUPS OF SMALL LIGHTTRANSMITTING AREAS, EACH GROUP BEING MADE UP OF CONGRUENT AREAS, SAIDLIGHT TRANSMITTING AREAS ADJOINING ONE ANOTHER AND THE AREAS ON THEEDGES OF THE GROUPS IN PARTS OF THE SCREEN OTHER THAN THE EDGES THEREOFADJOINING AREAS ADJACENT GROUPS, EACH GROUP OF AREAS HAVING SUB-GROUPSOF AREAS WITH THE PHOTOGRAPHIC DENSITY OF EACH AREA IN A SUB-GROUP BEINGIDENTICAL TO THE PHOTOGRAPHIC DENSITY OF THE OTHER AREAS IN THESUBGROUP, THE DENSITIES OF THE AREAS IN THE RESPECTIVE SUBGROUPS BEINGDIFFERENT FROM THE DENSITY OF THE AREAS IN EACH OF THE OTHER SUB-GROUPSWITHIN SAID GROUP OF AREAS, AND THERE BEING A SUB-GROUP CORRESPONDING TOEACH PHOTOGRAPHIC DENSITY IN A SERIES OF PHOTOGRAPHIC DENSITIES, THEAREAS OF EACH SUB-GROUP BEING DISTRIBUTED OVER THE TOTAL AREA OF THECORRESPONDING GROUPS IN A REGULAR HOMOGENEOUS OPTICAL PATTERN AND THEAREAS OF EACH SUB-GROUP WITHIN A GROUP TOGETHER WITH THE AREAS OF THEOTHER SUB-GROUPS WITHIN THE GROUP HAVING A LOWER PHOTOGRAPHIC DENSITYFORMING A REGULAR HOMOGENEOUS OPTICAL PATTERN, AND CHEMICALLY TREATINGTHE BASE TO MAKE A PRINTING MEMBER THEREOF.